It is a common practice in biological experimentation to separate proteins and nucleic acids, e.g., DNA, for analytical and preparative purposes using electrophoresis. During electrophoresis, molecules are separated according to their relative mobilities through a polymer network or gel under the influence of electric potentials.
Electrophoresis is performed by applying one or more samples to one end of a thin slab of the gel, e.g. hydrogel matrix such as polyacrylamide or agarose, and an electric potential is applied to the gel for a certain period of time inducing charged components of the samples to move in parallel directions for various distances. The migration distances depend on the relative mobilities of the components which in turn depend on their sizes and chemical natures. During or after the period of electrophoresis, the location of the various components in the gel are determined using autoradiography, fluorescent detection, or any other well known means for identifying the components.
Most users prepare gels as they are needed, i.e., handcast gels, rather than purchase commercially prepared gels, i.e., pre-cast gels, for reasons of experimental flexibility and cost. Preferably, polymerization or casting of gels is performed utilizing a liquid-tight mold or cassette. This is especially critical for polymerizing polyacrylamide if the gels are formed as thin, vertical slabs. A leak during polymerization may result in a gel that is too short to be of any use.
The most commonly-used vertical gel cassette comprises a pair of rectangular glass plates, i.e., an inner and an outer plate, each having a top edge, a bottom edge and sides. The plates are spaced apart by spacers along the sides of the plates, between the inner and outer plates. The spacers comprise flat, narrow strips of sealing material. The plates and spacers define a an internal volume of the cassette with an open top edge and open bottom edge. The inner plate often has a lower top edge than the outer plate to facilitate electrical contact between the top edge of the gel and an electrode via a buffer contained in an upper buffer chamber.
Vertical gel cassettes are made liquid-tight by securely clamping the glass plates against the spacers along the sides of the cassette and sealing the bottom edge of the cassette with separate sealing mechanisms. The bottom edge is sealed by any of a number of means such as inserting a solid bottom spacer, applying a thin layer of rapid-gelling material such as agarose to the bottom, sealing the bottom with a water-resistant adhesive tape, or pressing the bottom edge against an elastomeric surface.
Once the cassette has been assembled and the bottom sealed, a mixture of monomer, catalyst and buffer are introduced into the cassette and allowed to polymerize, i.e., the gel is cast. A well-forming comb can be inserted in the top edge of the mixture so that multiple individual test sample wells are formed during polymerization.
After polymerization, the separate sealing mechanisms are removed from the bottom edge of the cassette to provide electrical access to the bottom surface of the gel, and the well-forming comb is removed from the top edge of the gel.
The gel cassette is then transferred to an electrophoresis apparatus and electrically coupled with an upper electrode through a buffer medium contained in the upper buffer chamber and with a lower electrode through a buffer medium contained in a lower buffer chamber or tank.
Using conventional techniques, the sealing and casting are performed in a separate apparatus and/or location from the apparatus and location in which the electrophoretic separation is performed. Performing the sealing and casting in a separate apparatus and/or location complicates the sealing, casting, and electrophoresis procedure because the gel cassette has to be transferred from one holding apparatus and/or location to another and numerous separate parts related to the sealing, casting, and electrophoresis must be managed.